Emotion Regulation in Rheumatoid Arthritis presents research on the role of emotion regulation in relation to health of patients with rheumatoid arthritis. A chronic disease such as rheumatoid arthritis has consequences for many aspects of health. The way persons handle their emotions in daily life may influence how they perceive their health and perhaps, through physiological processes, the disease itself. Emotion regulation refers to the processes by which individuals influence which emotions they have, when they have them, and how they experience and express these emotions. This thesis examines models of the interaction between emotion regulation, physiological processes, disease activity, and health, as well as the development and outcome of an intervention aimed at emotional disclosure. In 335 patients with rheumatoid arthritis, different concepts of emotion regulation were summarized into four main categories: ambiguity, which is a combination of difficulty identifying and describing emotions and being ambivalent about the way one generally expresses emotions; control, which is the more or less intentional control of emotions and being a rational person; orientation, which represents being emotionally oriented and experiencing emotions intensely; and, expression of both positive and negative emotions towards others. It was cross-sectionally and prospectively shown that emotion regulation is related to quality of life, to the stress hormone noradrenaline, and to a decrease in psychological well-being and experienced symptoms, but not to disease activity. A negative role in perceived health of patients with rheumatoid arthritis is especially suggested for ambiguity and orientation. There were no indications that the relationships between emotion regulation and perceived health were due to changes in physiological parameters. More and stronger relationships were found between emotion regulation and perceived health and physiological functioning for women than for men. This supports the usefulness of sex-sensitive approaches in health care. An intervention aimed at expression of emotions (emotional disclosure) might influence health beneficially. A critical theoretical review and a randomized controlled trial of emotional disclosure moderate a too optimistic view of the potential of emotional disclosure interventions in clinical populations. An emotional disclosure intervention was not found to be effective in improving perceived health or clinical outcome. Our study does not offer convincing evidence for implementation of emotional disclosure interventions as an adjunct to regular health care of patients with rheumatoid arthritis. Potentially, the intervention may be useful for individual patients, but it is currently insufficiently clear which patients will profit. Generally, the findings of this thesis underscore that emotions and the way emotions are regulated are important in everyday life and in the adaptation to disease. Although researchers increasingly acknowledge the emotional aspects of rheumatoid arthritis, patients participating in the studies stated that they still perceive barely any attention for emotions and how they should handle them in regular health care. This thesis showed that emotion regulation and health are so closely intertwined, that it is worthwhile for health professionals to not only pay attention to the somatic aspects of the disease, but also to the accompanying emotions and emotion regulation in rheumatoid arthritis